The harsh conditions of desert ecosystems
has promoted the evolution of a complex set of
relations among desert organisms, a surprising
number of which are positive interactions
(Cloudsley-Thompson 1996). Desert shrubs in
general and woody legumes in particular, create
microhabitats that are critical for the survival of
other species. Small animals seek the shade of
desert trees and shrubs, birds find refuge and
nesting sites in their canopies, and many small
plants recruit their juveniles under the nitrogenrich
canopy of desert woody legumes such
as acacias, carobs, and mesquites. Because
of their crassulacean acid metabolism, desert
succulents such as agaves, aloes, and cacti are poor thermoregulators as young seedlings,
and cannot survive the harsh ground-level
midday temperatures. For this reason, they can
successfully germinate and establish only under the
protective shade of shrubby "nurse plants" that act
as true cornerstone species in desert conservation
(Figure 1.16). If the desert trees and shrubs are cut,
all the accompanying biota soon disappears.

Additionally, many desert plants have very specific
requirements in terms of their pollinators and
seed dispersers (Figure 1.17). Although some
desert ephemerals are truly unspecific in their
requirements and produce thousands of seed
with only wind-pollination, the slow-growing
desert perennials are frequently highly specialized
in their reproductive habits, and depend strictly
on co-evolved animals to help them out in their
sexual and reproductive processes. Many African
cactoid plants (euphorbs and asclepias) produce
foul-smelling flowers that attract carrion insects
as pollinators. New World giant cacti and agaves
produce sugar-rich nocturnal flowers that engage
the pollinating services of nectar-feeding bats, while
the sweet pulp of their fruits lures birds to disperse the seeds miles away. The red tubular flowers of
many desert shrubs attract hummingbirds and
giant sphinx-moths.